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(11) | EP 0 805 949 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
MEASURING LAYER THICKNESS USING BACKSCATTERING OF HIGH ENERGY PHOTONS SCHICHTDICKENMESSUNG, DIE RÜCKSTREUUNG VON HOCHENERGISCHEN PHOTONEN VERWENDET UTILISATION DE LA RETRODIFFUSION DE PHOTONS A HAUTE ENERGIE POUR MESURER L'EPAISSEUR DE COUCHES |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
FIELD OF THE INVENTION
[0001] The present invention relates to an in situ and non-destructive method and device for measuring the thickness of layers on substrates using backscattering of high energy photons.
BACKGROUND OF THE INVENTION
[0002] The ability to measure, nondestructively and in situ the thickness of growing thin films is very advantageous in many industrial applications. For example, it is important to be able to monitor the thickness of paint being sprayed on cars, trucks or aircraft during production. The costs associated with painting vehicles, particularly in assembly line production is quite significant so that applying too thick a paint layer has serious economic repercussions. Alternatively, if there is too thin a paint layer this may result in the vehicle having to be repainted.
[0003] Post production painting of vehicles generally involves applying three distinct layers comprising a primer coating or layer applied directly to the metal substrate, a base coating containing the pigment applied on top of the primer coating and a clear coating applied on top of the base coating. The total thickness of these layers is about 0.05 mm to 0.10 mm with about half of the total thickness being due to the top clear coat. It is preferable that each layer be of uniform thickness and manufacturers are particularly concerned about controlling the thickness of the base coat; however the base coat is the thinnest layer (about 0.01 mm) which makes it very difficult to control its thickness.
[0004] There are several known ways of estimating the average thickness of the paint layers. One way is to simply weigh the paint used to cover a certain area and, knowing the mean density of the paint, calculate the average thickness which is generally expressed in units of mg/cm2, known as the "areal density." Disadvantages of this and similar techniques is it is not an in situ technique, it is very labour intensive and does not give any information about the uniformity of the layers.
[0005] Another method and device for measuring paint thickness is disclosed in EP-A-0 380 226. This method relies upon irradiating a coating with x-rays and measuring fluorescent x-rays and Rayleigh peaks from the spectrum of scattered x-rays. There are several drawbacks to the method and device. The device relies upon use of an expensive LO-AX hyperpure germanium detector. The method only works by adding a non-radioactive fluorescent label of atomic number greater than 20 and the fluorescent label must be correlated with the radiation source due to correlation between fluorescence efficiency and energy of the x-rays. The method involves measuring the ratio of fluorescent x-rays and Compton rays and the ratio of Compton and Rayleigh yields. The fact that this method of measuring paint thickness is dependent upon adding a fluorescent label is a major drawback since compatibility of the paint and label material must be taken into account. In addition, it is not practicable or economic to add labels to paints in large scale paint applications such as in painting of automobiles. In order to measure paint thickness with useful precision by using the fluorescent x-rays of additives, it is necessary to add several parts per thousand of label material. Such concentration is difficult to maintain as a homogeneous mixture and it can also degrade the weather resistance of the paint. Therefore it would be very advantageous to provide a method of measuring paint thickness which avoids the need for adding fluorescent additives to the paint.
[0006] At present there is no single, reliable, economic method for accurate, in-situ and nondestructive monitoring of paint thickness as it is being applied to substrates. X-ray backscattering is one method which shows promise as a technique for estimating film thicknesses; however, this technique has severe limitations. A simplified model used in considering backscattering of x-rays from a paint layer on a metal backing is based on two assumptions: 1) the x-rays interact with the paint layer only by the mechanism of Compton scattering and because the total Compton scattering cross-section is almost exactly proportional to the mass, the backscattered x-ray intensity should be proportional to the mass/unit area of the paint layer over a broad range of x-ray energies; and 2) that x-rays penetrating through to the steel backing or substrate are fully attenuated or absorbed in the substrate and not back scattered.
[0007] The assumption that the intensity of the backscattered x-rays from the paint is almost exactly proportional to the paint thickness over a broad range of x-ray energies usually holds because the paint layer is so thin and comprised of elements of low atomic number. The model breaks down generally because of the assumption that the metal panel is a perfect absorber over a broad range of energies. This will be more fully discussed below but this drawback has severely limited the application of x-ray backscattering as a viable in situ technique.
[0008] A method of measuring thickness of individual sheets on a multiple-strip rubber calender is disclosed in Proceedings Of The Annual Conference Of Electrical Engineering Problems In The Rubber And Plastics Industries, Akron, Apr. 15-16, 1991, Institute Of Electrical And Electronics Engineers, pages 73-75, XP 000299130 Bates J.R. et al. "Gamma Backscatter Thickness Measurement For Control of Multiple-Strip Rubber Calenders. The method and device disclosed in this paper suffers from several disadvantages. First, the method and device are directed to measuring the thickness of a free standing single layer and not a system comprising one or more layers in which contributions from other layers may be present. The radioactive source has a strength of about 150 mCi which is a very strong source. The source holder is embedded directly into the Nal crystal so that the holder must be very large and thick to block very high levels of background x-rays from the source.
[0009] An axially symmetric gamma-ray backscatter system is disclosed in Nuclear Instruments and Methods in Physics Research A299 (1990) 377-381; Innes K. MacKenzie, "An Axially Symmetric Gamma-Ray Backscatter System". The device disclosed therein provides a spectrometer for measuring Compton profiles with high efficiency. Similar to the device for measuring paint thickness disclosed in EP-A-0 380 226 the spectrometer uses an expensive LO-AX germanium detector.
SUMMARY OF THE INVENTION
[0010] The present invention provides a non-destructive, in-situ method of measuring thickness of layers comprising elements with low atomic numbers coated or disposed on a substrate during or after deposition on the substrate.
[0011] The present invention provides a method for measuring thickness of a coating comprising low atomic numbers formed on a surface of a metal substrate such as aluminum, titanium, iron, copper, zinc and combinations thereof, steel, galvanized steel and other metals having atomic numbers from 13 to 32, the method comprising the steps of providing a radioactive source in a source holder, positioning said source holder in opposing relation to a surface of a metal substrate and determining the thickness of coating from an intensity of photons backscattered from coating incident on a photodetection means, the source holder being fabricated of a sufficiently dense and thick material so that the photodetection means is shielded from radiation from the source, and the radioactive source being selected to produce photons having energies in the range from about 14 keV to about 25 keV, sufficiently high so that Compton scattering from the coating is enhanced over Compton and Rayleigh scattering from the metal substrate but low enough to provide contrast between photoelectric absorption in the coating and the metal substrate.
[0012] The present invention provides a method for controlled application of paint to a surface of a metal substrate such as aluminum, titanium, iron, copper, zinc and combinations thereof, steel, galvanized steel and other metals having atomic numbers from 13 to 32, the method comprising the steps of: a) providing a radioactive source in a source holder, the radioactive source being selected to produce photons having energies in the range from about 14 keV to about 25 keV sufficiently high to provide dominance of Compton scattering from the paint coating over Compton and Rayleigh scattering from the substrate but low enough to provide sufficient contrast between photoelectric absorption in the paint coating and the metal substrate, the source holder being fabricated of a sufficiently dense and thick material so that a photodetection means is shielded from radiation from the source; b) applying paint to said surface and positioning said source holder in opposing relation to the surface of the metal substrate being painted and measuring an intensity of backscattered secondary photons from the paint coated metal substrate; and c) processing said intensity of backscattered photons and calculating therefrom a thickness of the paint coating, comparing said thickness to a preselected paint thickness and if the thickness of the paint coating is less than said preselected paint thickness repeat step b) otherwise terminate applying paint to said surface.
[0013] The present invention provides an apparatus for controlling application of a paint coating to a metal surface of a vehicle, the metal being one of aluminum, titanium, iron, copper, zinc and combinations thereof, steel, galvanized steel and other metals having atomic numbers from 13 to 32, comprising: a plurality of probes mounted on a frame, each probe including a radioactive source located in a source holder and a photodetection means located behind each source holder for measuring an intensity of backscattered photons, the radioactive source being selected to produce photons having energies in the range from about 14 keV to about 25 keV, high enough to provide dominance of Compton scattering from the paint coating over Compton and Rayleigh scattering from the metal substrate but low enough to provide effective contrast between photoelectric absorption in the paint coating and the metal substrate, the source holder being fabricated of a sufficiently dense and thick material so that a photodetection means is shielded from radiation from the source; and a painting device for dispensing paint and computer control means connected to said painting device and said probes wherein each probe forms a feedback element for computer control of the amount of paint discharged by said painting device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The method of measuring thickness in inhomogeneous layered systems according to the present invention will now be described, by way of example only, reference being had to the accompanying drawings, in which:
Figure 1 is a longitudinal section of a radioactive source, holder and detector for measuring thicknesses of layers on substrates according to the present invention;
Figure 2a illustrates the backscattered x-ray energy spectra for the K-shell x-rays of silver from a copper target;
Figure 2b illustrates the backscattered x-ray energy spectra for the K-shell x-rays of silver from low density polyethylene; and
Figure 3 shows the ratio of the reflection of silver K-shell x-rays from thick specimens of various materials to that from a thick block of low density polyethylene;
Figure 4 is a plot of relative x-ray albedos versus x-ray energy and atomic number Z;
Figure 5 is a longitudinal section of a probe including radioactive source, holder and detector for measuring thickness of paint layers on substrates according to the present invention;
Figure 6 illustrates the use of a plurality of the probes of Figure 5 to measure the thickness of paint layers applied to a vehicle on an assembly line; and
Figure 7 is a plot illustrating the variation of count rate versus time due to solvent evaporation from the freshly spray painted surface.
DETAILED DESCRIPTION OF THE INVENTION
A) BASIC CONFIGURATION OF SOURCE-DETECTOR-TARGET SYSTEM
[0015] The basic design and geometric arrangement of an axially or cylindrically symmetric detector-source geometry constructed in accordance with an aspect of the present invention will be discussed first followed by a description of preferred embodiments for the specific application of measurement of paint thickness on metal panels such as vehicles. The preferred embodiments of this invention illustrated in the drawings are not intended to be exhaustive or to limit the invention to the precise form disclosed so that the applications cited are exemplary in nature and are not intended to limit the scope of the invention. The particular applications disclosed herein are chosen to describe the principles of the invention and its applicable and practical use to thereby enable others skilled in the art to best utilize the invention.
[0016] Referring to Figure 1, a longitudinal section of a detector-source-target arrangement constructed in accordance with the present invention is shown at 20. A scintillation detector 22 includes a thin (1.0 mm) Nal(TI) scintillator 24 housed in an aluminum cylinder (not shown) of 5.08 cm external diameter and 15.24 cm in length which also houses a photomultiplier 26. A protective covering 28 such as MYLAR extends across the scintillator. A lead shielding 30 is provided around the sides of detector 22 to minimize multiple scattering from nearby objects.
[0017] A source holder 32 is provided with a longitudinal cavity 34 extending partly therethrough for holding a radioactive source 38. Holder 32 is shown as being tubular with a radius R1 and cavity 34 defines a detector axis 36. Holder 32, also referred to as an absorber post, is fabricated of a sufficiently thick and dense material so that primary radiation from source 38 is blocked or absorbed before hitting detector 22 below the source.
[0018] Radioactive source 38 is preferably a commercially available sealed source of x-rays or γ-rays typically 3.0 mm in length and diameter. Source 38 sits on a threaded stud 42 threadable movable in cavity 34. Source 38 sits at an adjustable depth Z1 below the top surface 40 of holder 32. The geometry and structure of holder 32 are such that with a source 38 in the holder, the primary radiation moves upwards in a cone whose half angle is adjustable by the depth Z1. The area of a target 46 (shown as two contiguous layers 48 and 50) spaced a distance Z2 from surface 40 irradiated by the source is determined by both the half angle and the spacing Z2. Target 46 is therefore located with respect to the source 38 and detector 22 array so that it intercepts the cone of primary photons emanating from the source which may interact with the target in several ways to produce a variety of secondary radiation.
[0019] The diameter of source holder 32 may vary from about 5 mm to about 22 mm and the holder may be fabricated of gold or other suitable high density material depending on the application. For example, platinum, tungsten, silver, molybdenum, lead and tantalum may all be used as materials for the source holder. The detector assembly may optionally include an iris 54 defining an aperture 56 and having an inner radius R2 symmetrically disposed with respect to source holder 32. Iris 54 is formed of a material which acts to absorb x-rays and γ-rays. Therefore, the backscattered photons can reach detector 22 only by passing through the annulus defined by radius R1 of the source holder and R2 of iris 54. Holder 32 blocks primary radiation from the source impinging on the detector. Iris 54 is optional since holder 32 is preferably made of a material having an effective density and shape to substantially block photons from the source from impinging on the photodetector and so is not required for some applications described herein.
[0020] The variables of the detector-source-target system include the dimensions R1, R2, Z1, Z2, the presence or absence of iris 54 and the choice of radioactive source 38. There are in principle several radioactive sources which may be used with the choice of radioactive source being dependent on the specific application. The preferred radioactive sources used in the present invention include 241Am and 109Cd and 57Co. Further details of the detector-source configuration are found in Innes K. MacKenzie, An Axially Symmetric Gamma-Ray Backscatter System For Dumond Spectrometry, Nuclear Instruments and Methods in Physics Research A299 (1990) 377-381.
B) MEASUREMENT OF PAINT THICKNESS ON SUBSTRATES USING X-RAY BACKSCATTERING
[0021] A drawback to previous attempts to use x-ray backscattering for measuring paint thickness on for example vehicles on an assembly line using the soft 6 kiloelectron volts (keV) x-rays from 55Fe was strong absorption of the x-rays in the air gap between the probe and the paint layer. Sway of vehicles on an assembly line is unavoidable and so a minimum air gap of about 2.5 cm preferably exists between the probe and the surface being painted to avoid physical contact between the probe and panel being painted.
[0022] A more fundamental limitation of x-ray backscattering using low energy x-rays relates to the fact that typically Rayleigh scattering from the metal substrate has been assumed negligible and therefore ignored when interpreting the data. The inventor however has determined that Rayleigh scattering contributes significantly to the backscattered intensity when using soft x-ray photons of energy 6 keV produced by 55Fe. As discussed above, the simplest model used in considering backscattering of x-rays from a paint layer on a metal backing is based on the assumption that the x-rays are reflected by the paint layer only and x-rays penetrating through to the steel backing or substrate are fully attenuated or absorbed and not backscattered. In this model the x-rays interact with the paint layer only by the mechanism of Compton backscattering.
[0023] While the assumption that the intensity of backscattered x-rays from the paint is almost exactly proportional to the paint thickness over a broad range of x-ray energies is valid, the model breaks down with the assumption that the metal backing is a perfect absorber. Using steel as an example, it is always true that absorption has to compete with scattering in the steel. A good approximation to Compton scattering per atom is that it is proportional to the atomic number Z whereas photoelectric absorption is proportional to Z5 so the ratio is approximately proportional to Z4. Hence steel (iron) is a vastly better absorber than paint which comprises primarily carbon, hydrogen and other low-Z elements.
[0024] However, there is another important consideration. That is that the photoelectric absorption cross section varies roughly as the inverse 7/2 power of the x-ray energy. To illustrate the importance of this, consider the mass attenuation coefficients as a function of x-ray energy for copper given in Table I. Table I compares x-ray mass attenuation coefficients as a function of x-ray energies for several metals. At x-ray energies above 14 keV the mass attenuation coefficient varies fairly slowly with energy. That is because attenuation due to the photoelectric effect is weak in this regime and much of the attenuation is caused by the slowly varying Compton effect. At energies from about 8.95 to 13 keV the changes are very rapid and this corresponds to the region where photoelectric absorption dominates. At about 6 keV the attenuation in copper is about 23 times as great as at 34.92 keV. A similar situation applies to iron (not shown) so that, when considering only photoelectric absorption and Compton scattering one is led to the conclusion that in order to achieve the ideal of high absorption in the metal substrate very low x-ray energies are required.
[0025] This prior art model ignores the effect of Rayleigh scattering which in certain energy ranges may effectively compete or dominate photoelectric absorption and/or Compton scattering. At low energies such as 6 keV poor contrast between paint and steel is obtained due to the backscattered intensity being dominated by Rayleigh scattering instead of the expected Compton scattering.
[0026] The backscattered energy spectra for the K x-rays of Ag (22 and 25 keV) incident on copper are shown in Figure 2(a), and for polyethylene in Figure 2(b). The intensity vs. energy spectra for polyethylene (CH2)n, shows only two peaks A1 and A2 for polyethylene which are due to Compton scattering proving that it is an almost ideal Compton scatterer at 22 and 25 kev. When the same measurement is made on copper there are two additional Rayleigh peaks B1 and B2 that are more intense than the Compton peaks A1 and A2.
[0027] The dominance of Rayleigh scattering increases rapidly at lower x-ray energies. Therefore, for a paint film on a steel metal substrate and using an 55Fe source of x-rays of energy of approximately 6 keV, Compton scattering from the steel will be considered insignificant compared to Rayleigh scattering from the steel and so the contributions to the backscattered x-ray intensity are Compton scattering from the paint and Rayleigh scattering from the steel. Therefore the contrast between the backscattering from the steel substrate and the paint is destroyed and most of the intensity of the backscattered photons comes from the steel even with very thick paint layers so that low energy soft x-rays cannot be used reliably to measure paint thickness on steel substrates.
[0028] The method of the present invention is based on the fact that in order to measure the thickness of materials of low atomic numbers coated on substrates with high atomic numbers, one must use high energy photons in the appropriate energy range such that Compton scattering from the layers comprised substantially of elements of low atomic number Z competes successfully with the total of Compton and Rayleigh scattering from the substrate with high atomic number. The choice of options is very limited because there are few radioactive sources having a reasonable half-life that produce primary photons of suitable energy.
[0029] The influence of the underlying substrate material is best understood with reference to Figure 3 which shows the ratio of the reflection of silver K x-rays (22 and 25 keV) from very thick specimens of various materials to that from a thick block of low-density polyethylene. Hereinafter this relative reflectivity ratio is referred to as the albedo (whiteness). Paint is a low-Z mixture with an albedo close to 1.0. An ideal backing material would exhibit an albedo of zero but it is clear that this ideal is unattainable. The lowest albedo value for the K x-rays of silver is about 0.025 is obtained using a zinc substrate; i.e. zinc reflects about 1/40th as much as polyethylene. Iron exhibits an albedo slightly higher than zinc but the difference is significant as will be further discussed below.
[0030] Figure 4 illustrates a plot of relative x-ray albedos versus x-ray energy and atomic number Z which the inventor has discovered is most useful for selecting the best x-ray source for a given task. The best results are obtained for the largest contrast (i.e. ratio of albedos) between the low-Z paint and the backing material. For measuring paint layer thickness on galvanized steel the preferred sources will produce primary photons in the energy range of about 14 keV to about 25 keV with Figure 4 showing that a 109Cd source of the Ag K-shell primary photons of energy of 22 keV and 25 keV, is a most preferred source. For measuring the thickness of paint layers on steel panels, these Ag K-shell photons at 22 keV and 25 keV produce better contrast than the Np L photons from a 241Am source having an energy of 17 keV, the 14 keV photons from a 57Co source and substantially better contrast than the soft 6 keV x-rays from 55Fe.
[0031] Referring to Figure 5, the method for measuring thickness of a paint layer 60 on a substrate 62 involves positioning a probe 58, comprising detector 22 and source holder 32 containing a small x-ray source 38, at a distance of about 1 cm (or further depending on the anticipated sway) from panel 62 and measuring the intensity of the backscattered x-rays. Since paint layer 60 is much thinner than substrate 62 and because it is comprised of elements of low atomic number, the intensity of the x-rays backscattered by the paint will be proportional to the thickness of the paint.
[0032] The use of the 22 and 25 keV primary photons from 109Cd as part of the paint thickness monitoring device is also very advantageous because these photons are attenuated to a much lesser extent by air than the 6 keV photons of 55Fe. This is particularly problematic on production lines where side-to-side swaying of the vehicle occurs as it moves along the production line so that the probe 58 must be spaced at least 1 cm from the vehicle depending on the amount of sway.
[0033] The data for different colored paint layers on steel panels are summarized in Table II at the end of the description which displays the backscattered x-ray intensity from several General Motors panels each painted with a different color. The instrumentation used to obtain the results disclosed herein was produced by Ludlum Measurements Inc. (Sweetwater, Texas, USA) and the radioactive source used was a 109Cd source of Ag K x-rays (22 and 25 keV) having a strength of about 0.3 millicuries which produces a counting rate on steel panels of only about 500 cps. The last column in Table II was obtained by an ancillary measurement using a clear plastic sheet of thickness 0.10 mm having an areal density of 13.2 mg/cm2 adhered to a bare steel panel. This gave a ratio of 1.406 and served as a calibration for the paint layers.
[0034] The results in Table II are noteworthy for two reasons. First, the accuracy is about 2% for the estimate of areal density. Secondly, this required 100 seconds using the weak x-ray source but those skilled in the art will appreciate that it would for example require only 10 seconds with a stronger source of strength of 3 millicuries and only about 1 second with a 30 millicuries source and a fast counting system. As the source strength decreases the precision decreases (roughly as the square root of the inverse source strength). The operative upper limit of the source strength is determined by the counting rate the detection system can tolerate. Use of detectors with high counting efficiency allows weak, non-hazardous sources to be used.
[0035] In order to more fully illustrate the effect of substrate composition on backscattering intensity studies were conducted using a paint coating of 10 mg/cm2 on a) an aluminum panel; b) a steel panel; and c) a zinc panel. The electronic system employed can count at 5,000 counts/sec (cps) and the source strength is chosen to provide that rate. A system for aluminum would use a much weaker source than one for steel, and one for zinc would use a stronger source than for steel. Of the total counts, the fraction, f, contributed by the paint is (a) 0.057 for aluminum, (b) 0.258 for steel and (c) 0.304 for zinc.
[0036] If the counting rate is 5,000 cps for a time of t seconds the counts due to the paint is given by:
and the % of error in this count is:
If it is desirable to measure the intensity to a precision of 5% the counting times are (a) 48 seconds for the aluminum panel, (b) 2.1 seconds for the steel panel and (c) 1.5 seconds for the zinc panel.
[0037] To summarize, the background scattering from the panel interferes with the measurement in two ways. It forces the use of a weaker source in order to avoid saturating the counting system and secondly, it introduces statistical errors that force longer counting times to achieve a given precision. The fairly modest goal of 5% precision requires about a minute of counting on Al panels and 1% precision would require about 25 minutes.
[0038] The situation is much better with steel panels. In this case, a precision of 5% is obtained in about 2 seconds of counting and 1% precision in 1 minute. These figures can be reduced by about 30% by using galvanized iron (almost equivalent to zinc) if the time is a critical factor. Therefore, the present method is highly advantageous for assembly line painting of vehicles comprising a substrate of galvanized iron, shown in Figure 5 as a zinc layer 61 on an ferrous based substrate 62.
[0039] If it is desired to use the present counting system near to its design limits then stronger x-ray sources may be employed. Furthermore, the present counting system is an extremely simple, conservative design. It is well within the capabilities of currently available instrumentation to operate at a counting rate of 50,000 cps. Hence, where speed of analysis is essential, such as when the thickness monitoring system forms part of a feedback loop in a system controlling the painting operations, the system is readily adaptable to count at 100 times the rate used in acquiring the data for Table II. In other words, the counts acquired in 100 seconds can be obtained in 1 second. There would be substantially no sacrifice in the accuracy of the data but the high-speed instrumentation could cost several times as much as the low-speed system.
[0040] Referring to Figure 6, there is shown generally at 70 a plurality of probes 58 mounted on a frame 72 forming part of an assembly line for painting a vehicle 74. The output of each probe 58 is input into a computer 76 by wires 78 which may be used for monitoring paint thickness during painting and which may be used as a feedback element for controlling the painting process. In order to avoid the problem of the probes being coated by paint during the painting operation they may be disposed along the painting line between painting stations and suitably shielded. Alternatively, the probes could be positioned within the painting stations in retractable, shielded enclosures so that the painting operation can be interrupted and the probes moved into position and unshielded to measure the paint thickness.
[0041] The present method may be used to probe paint thickness during the application process before it has dried as well as being used as a probe for the dried paint. If the probe is used as part of a feedback element in a painting process, then a calibration must be used to compensate for evaporation of solvents from the paint after it is applied to the substrate. Paints applied using spraying include a solvent carrier which comprise low-Z elements. The solvent component could exceed 50% of the total volume of the paint mixture. The solvent will also contribute to the backscattered intensity so that each mixture being used would have to be characterized. In other words the x-ray measurement of paint thickness disclosed herein does not distinguish between paint and solvent per se since both are comprised substantially of elements of low atomic number, rather, the method measures the total areal density of matter deposited onto the substrate surface.
[0042] Figure 7 shows the decrease in counting rate versus time after spraying for one particular commercially available spray paint. The initial rate of decay is very rapid and is followed by an approximately linear decay for about 2 hours, corresponding to a linear loss of solvent from the layer. After about 16 hours the readings become very stable suggesting solvent evaporation has substantially ceased. The drying rate (solvent evaporation rate) depends on ambient temperature and air flow over the painted surface. Therefore, in order to use the method for monitoring paint thickness shortly after application to the substrate, the effect on drying rate of composition of the spray paint, ambient temperature, rate of air flow over the substrate and the like is required.
[0043] The method of measuring paint thickness is most advantageous when the paint is comprised substantially of low-Z atomic elements. However, the method is still advantageous with paints having some high-Z atomic elements present. For example, some paints include titanium dioxide powder. The thickness of these types of paint can be measured using the present technique as long as the impact of the higher atomic element components is accounted for in the calibration procedure.
[0044] Referring again to Figures 3 and 4, those skilled in the art will appreciate that when Ag K x-rays produced by 109Cd are used, zinc exhibits the lowest albedo and hence the best contrast. The thickness of paint layers on other substrates such as ungalvanized steel, Fe, Ti, Si and A1 and the like may also be measured using the present method but will exhibit lower contrast than with galvanized steel having a zinc coating so that weaker sources and longer counting times are required.
[0045] Therefore, while the present invention has been described and illustrated with respect to the preferred embodiments for measuring the thickness of paint layers, it will be appreciated that numerous variations of these embodiments may be made depending on the application without departing from the scope of the invention as defined in the appended claims.
| Energy (keV) | Mass attenuation coefficients (cm2g-1) |
| Copper (Z=29) | |
| 6.00 | 131.0 |
| 7.00 | 73.6 |
| 8.00 | 50.3 |
| 8.59 | 40.7 |
| 8.75 | 40.3 |
| 8.83 | 39.5 |
| 8.95 | 306.0 |
| 9.00 | 290.0 |
| 9.12 | 278.0 |
| 9.30 | 262.0 |
| 10.00 | 206.0 |
| 11.00 | 159.0 |
| 13.20 | 98.5 |
| 14.23 | 83.8 |
| 15.40 | 67.6 |
| 16.62 | 53.4 |
| 18.20 | 41.9 |
| 20.39 | 30.5 |
| 22.34 | 24.1 |
| 24.89 | 17.6 |
| 27.99 | 12.8 |
| 34.92 | 6.3 |
| 39.86 | 5.6 |
| PANEL NO. | COLOR | INTENSITY c/100sec | RATIO w.r.t. bare panel | TOTAL PAINT THICKNESS mg/cm2 |
| 1 | bare | 37,413 | 1.000 | 0 |
| 2 | black | 50,926 | 1.361 | 11.7 ± .08 |
| 3 | white | 53,823 | 1.439 | 14.3 ± .021 |
| 4 | maroon | 50,403 | 1.347 | 11.3 ± .018 |
| 5 | red | 53,905 | 1.441 | 14.3 + .021 |
| 6 | sandy | 50,608 | 1.353 | 11.5 ± .018 |
providing a radioactive source (38) in a source holder(32), positioning said source holder (38) in opposing relation to a surface of a metal substrate and determining the thickness of coating (50) from an intensity of photons backscattered from coating (50) incident on a photodetection means (22), the source holder (32) being fabricated of a sufficiently dense and thick material so that the photodetection means (22) is shielded from primary radiation from the source (38), the improvement in the method characterized by:
the radioactive source (38) being selected to produce photons having energies in the range from about 14 keV to about 25 keV, sufficiently high so that Compton scattering from the coating (50) is enhanced over Compton and Rayleigh scattering from the metal substrate (48) but low enough to provide contrast between photoelectric absorption in the coating (50) and the metal substrate (48).
a) providing a radioactive source (38) in a source holder (32), the radioactive source (38) being selected to produce photons having energies in the range from about 14 keV to about 25 keV sufficiently high to provide dominance of Compton scattering from the paint coating over Compton and Rayleigh scattering from the substrate but low enough to provide sufficient contrast between photoelectric absorption in the paint coating and the metal substrate, the source holder (32) being fabricated of a sufficiently dense and thick material so that a photodetection means (22) is shielded from radiation from the source (38);
b) applying paint to said surface and positioning said source holder (32) in opposing relation to the surface of the metal substrate (48) being painted and measuring an intensity of backscattered secondary photons from the paint coated metal substrate (48); and
c) processing said intensity of backscattered photons and calculating therefrom a thickness of the paint coating (50), comparing said thickness to a preselected paint thickness and if the thickness of the paint coating (50) is less than said preselected paint thickness repeat step b) otherwise terminate applying paint to said surface.
a) a plurality of probes (58) mounted on a frame (72), each probe (58) including a radioactive source (38) located in a source holder (32) and a photodetection means (22) located behind each source holder (32) for measuring an intensity of backscattered photons, the radioactive source (38) being selected to produce photons having energies in the range from about 14 keV to about 25 keV, high enough to provide dominance of Compton scattering from the paint coating over Compton and Rayleigh scattering from the metal substrate but low enough to provide effective contrast between photoelectric absorption in the paint coating and the metal substrate, the source holder (32) being fabricated of a sufficiently dense and thick material so that a photodetection means (22) is shielded from radiation from the source (38); and
b) a painting device (70) for dispensing paint and computer control means (76) connected to said painting device and said probes (58) wherein each probe forms a feedback element for computer control of the amount of paint discharged by said painting device (70).
Vorsehen einer radioaktiven Quelle (38) in einer Quellenhaltevorrichtung (32), Anordnen der Quellenhaltevorrichtung (38) in entgegengesetzter Beziehung zu einer Oberfläche eines Metallsubstrats und Bestimmen der Dicke einer Schicht (50) aus einer Intensität von von einer Schicht (50) rückgestreuten Photonen, welche auf eine Photoerfassungseinrichtung (22) auftreffen, wobei die Quellenhaltevorrichtung (32) aus einem ausreichend dichten und dicken Material hergestellt ist, so daß die Photoerfassungseinrichtung (22) von einer Primärstrahlung von der Quelle (38) abgeschirmt ist, wobei die Verbesserung des Verfahrens dadurch gekennzeichnet ist, daß
die radioaktive Quelle (38) derart gewählt ist, daß diese Photonen erzeugt, welche Energien in dem Bereich von etwa 14 keV bis etwa 25 keV aufweisen, die ausreichend hoch sind, um eine Compton-Streuung von der Schicht (50) gegenüber einer Compton- und Rayleigh-Streuung von dem Metallsubstrat (48) zu verbessern, jedoch niedrig genug sind, um einen Gegensatz zwischen einer photoelektrischen Absorption in der Schicht (50) und dem Metallsubstrat (48) zu liefern.
a) Vorsehen einer radioaktiven Quelle (38) in einer Quellenhaltevorrichtung (32), wobei die radioaktive Quelle (38) derart gewählt ist, daß diese Photonen mit Energien in dem Bereich von etwa 14 keV bis etwa 25 keV erzeugt, welche ausreichend hoch sind, um eine Dominanz der Compton-Streuung von der Lackschicht gegenüber einer Compton- und Rayleigh-Streuung von dem Substrat zu liefern, jedoch niedrig genug sind, um einen ausreichenden Gegensatz zwischen einer photoelektrischen Absorption in der Lackschicht und dem Metallsubstrat zu liefern, wobei die Quellenhaltevorrichtung (32) aus einem ausreichend dichten und dicken Material hergestellt ist, so daß eine Photoerfassungseinrichtung (22) von einer Strahlung von der Quelle (38) abgeschirmt ist;
b) Aufbringen von Lack auf die Oberfläche und Anordnen der Quellenhaltevorrichtung (32) in entgegengesetzter Beziehung zu der Oberfläche des sich in Lackierung befindlichen Metallsubstrats (48) und Messen einer Intensität rückgestreuter Sekundärphotonen vom lackbeschichteten Metallsubstrat (48); und
c) Verarbeiten der Intensität rückgestreuter Photonen und Berechnen anhand davon einer Dicke der Lackschicht (50), Vergleichen der Dicke mit einer vorausgewählten Lackdicke und, falls die Dicke der Lackschicht (50) geringer ist als die vorausgewählte Lackdicke, Wiederholen des Schritts b), ansonsten Beenden der Aufbringung von Lack auf die Oberfläche.
a) eine Vielzahl von Sonden (58), die an einem Rahmen (72) angebracht sind, wobei jede Sonde (58) eine radioaktive Quelle (38), welche in einer Quellenhaltevorrichtung (32) angeordnet ist, und eine Photoerfassungseinrichtung (22) umfaßt, welche hinter jeder Quellenhaltevorrichtung (32) zum Messen einer Intensität rückgestreuter Photonen angeordnet ist, wobei die radioaktive Quelle (38) derart gewählt ist, daß diese Photonen mit Energien in dem Bereich von etwa 14 keV bis etwa 25 keV erzeugt, welche hoch genug sind, um eine Dominanz der Compton-Streuung von der Lackschicht über die Compton- und Rayleigh-Streuung von dem Metallsubstrat zu liefern, jedoch niedrig genug, um einen effektiven Gegensatz zwischen einer photoelektrischen Absorption in der Lackschicht und dem Metallsubstrat zu liefern, wobei die Quellenhaltevorrichtung (32) aus einem ausreichend dichten und dicken Material hergestellt ist, so daß eine Photoerfassungseinrichtung (22) von einer Strahlung der Quelle (38) abgeschirmt ist; und
b) eine Lackiervorrichtung (70) zum Aufbringen von Lack und eine Computersteuereinrichtung (76), welche mit der Lackiervorrichtung und den Sonden (58) verbunden ist, wobei jede Sonde ein Rückkopplungselement zur Computersteuerung der Menge von durch die Lackiervorrichtung (70) abgegebenem Lack bildet.
fournir une source radioactive (38) dans un réceptacle (32) pour la source, positionner ledit réceptacle (38) pour la source en opposition à une surface d'un substrat métallique et déterminer l'épaisseur du revêtement (50) à partir de l'intensité des photons rétrodiffusés à partir du revêtement (50) tombant sur des moyens (22) de photodétection, le réceptacle (32) pour la source étant fabriqué en un matériau suffisamment dense et épais pour que les moyens (22) de photodétection soient protégés contre le rayonnement primaire provenant de la source (38), les perfectionnements du procédé étant caractérisés par le fait que :
la source radioactive (38) est choisie de façon à produire des photons ayant des énergies qui se situent dans la plage d'environ 14 keV à environ 25 keV, suffisamment élevée pour que la diffusion Compton provenant du revêtement (50) soit augmentée par rapport à la diffusion Compton et Rayleigh provenant du substrat métallique (48), mais suffisamment basse pour fournir un contraste entre l'absorption photoélectrique dans le revêtement (50) et le substrat métallique (48).
a) fournir une source radioactive (38) dans un réceptacle (32) pour la source, la source radioactive (38) étant choisie de façon à produire des photons ayant des énergies se situant dans la plage d'environ 14 keV à environ 25 keV suffisamment élevée pour fournir une dominance de l'effet Compton provenant du revêtement de la peinture par rapport à la diffusion par effet Compton et Rayleigh provenant du substrat, mais suffisamment faible pour fournir un contraste suffisant entre l'absorption photoélectrique dans le revêtement de peinture et le substrat métallique, le réceptacle (32) pour la source étant fabriqué en un matériau suffisamment dense et épais pour que les moyens (22) de photodétection soient protégés du rayonnement de la source (38) ;
b) appliquer la peinture à ladite surface et positionner ledit réceptacle (32) pour la source en opposition à la surface du substrat métallique (48) qui est peint et mesurer l'intensité des photons secondaires rétrodiffusés à partir de la peinture dont est revêtu le substrat métallique (48) ; et
c) traiter ladite intensité des photons rétrodiffusés et calculer à partir de là une épaisseur pour le revêtement (50) de peinture, comparer ladite épaisseur à une épaisseur de peinture présélectionnée et si l'épaisseur du revêtement de peinture (50) est inférieure à ladite épaisseur de peinture présélectionnée, répéter l'étape b) ou en cas contraire terminer l'application de la peinture à ladite surface.
a) une pluralité de sondes (58) montées sur un bâti (72), chaque sonde (58) comprenant une source radioactive (38) située dans un réceptacle (32) pour la source et des moyens (22) de photodétection situés derrière chaque réceptacle (32) pour la source pour mesurer l'intensité des photons rétrodiffusés, la source radioactive (38) étant choisie de façon à produire des photons ayant des énergies se situant dans la plage d'environ 14 keV à environ 25 keV, suffisamment élevée pour fournir une dominance de l'effet Compton provenant du revêtement de la peinture par rapport à la diffusion par effet Compton et Rayleigh provenant du substrat, mais suffisamment faible pour fournir un contraste suffisant entre l'absorption photoélectrique dans le revêtement de peinture et le substrat métallique, le réceptacle (32) pour la source étant fabriqué en un matériau suffisamment dense et épais pour que les moyens (22) de photodétection soient protégés du rayonnement de la source (38) ; et
b) un dispositif (70) de peinture pour fournir de la peinture et des moyens de commande par ordinateur (76) reliés audit dispositif de peinture et auxdites sondes (58) où chaque sonde forme un élément à rétroaction pour la commande de l'ordinateur de la quantité de peinture fournie par ledit dispositif de peinture (70).